how to check if a hash is empty in perl
PerlHashPerl Problem Overview
I use the following code to check if the hash is empty. Is there a better method and is this safe to use?
if (!keys %hash) { print "Empty";}
Perl Solutions
Solution 1 - Perl
if (%hash)
Will work just fine.
From perldoc perldata:
> If you evaluate a hash in scalar context, it returns false if the hash > is empty. If there are any key/value pairs, it returns true; more > precisely, the value returned is a string consisting of the number of > used buckets and the number of allocated buckets, separated by a > slash.
Solution 2 - Perl
There was a bug which caused tied hashes in scalar context to always return false. The bug was fixed in 5.8.5. If you're concerned with backwards compatibility that far back I would stick with if( !keys %hash ). Otherwise use if( !%hash ) as recommended by others.
Solution 3 - Perl
Simpler:
if (!%hash) {
print "Empty";
}
! imposes a scalar context, and hash evaluated in a scalar context returns:
- false if there are zero keys (not defined in the documentation but experimentally returns
0)
Depending on the version of Perl, either of the following:
-
A string signifying how many used/allocated buckets are used for >0 keys, which will of course be NOT false (e.g. "3/6"). (Non-empty string evaluate to true)
-
The number of keys in the hash (as explained in
perldata: "As of Perl 5.25 the return was changed to be the count of keys in the hash. If you need access to the old behavior you can use "Hash::Util::bucket_ratio()" instead.")
Solution 4 - Perl
"Better" is a subjective term. However I would argue that code that is easier to understand can be described as "better". For this reason I conclude that !keys %hash is better, because everybody writing perl code will know what this code does and that it works. !%hash is something at least I would have to look up to ensure if it really works or only looks like it would work. (The reason being that the return value of a hash in scalar context is rather confusing while an arrays behavior in scalar context is well known and often used.)
Also, !keys %hash is safe.
So no, there is no better or safer way to check if a hash is empty.